Abstract
We have studied the optical properties of $WSe_2$ monolayers (ML) by means of photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopy at room temperature and as a function of hydrostatic pressure up to ca. 12 GPa. For comparison the study comprises two cases: A single $WSe_2$ ML directly transferred onto one of the diamonds of the diamond anvil cell and a $WSe_2$ ML encapsulated into hexagonal boron nitride (hBN) layers. The pressure dependence of the A and B exciton, as determined by PL and PLE, respectively, is very different for the case of the bare $WSe_2$ ML and the $hBN/WSe_2-ML/hBN$ heterostructure. Whereas for the latter the A and B exciton energy increases linearly with increasing pressure at a rate of 3.5 to 3.8 meV/GPa, for the bare $WSe_2$ ML the A and B exciton energy decreases with a coefficient of -3.1 and -1.3 meV/GPa, respectively. We interpret that this behavior is due to a different stress situation. For a single ML the stress tensor is essentially uniaxial with the compressive stress component in the direction perpendicular to the plane of the ML. In contrast, for the substantially thicker $hBN/WSe_2-ML/hBN$ heterostructure the compression is hydrostatic. The results from an analysis of the pressure dependence of the frequency of Raman active modes comply with the interpretation of having a different stress situation in each case.
 Reviewed by: A. San Miguel, Institut Lumière Matière, Université de Lyon, France; Edited by: J. S. Reparaz
Highlights
Monolayers (MLs) of group-VI transition metal dichalcogenides (TMDs), MX2 with M=Mo, W and X= S, Se, Te, have emerged as fascinating twodimensional (2D) semiconductors due to remarkable properties that differ from the bulk
We have studied the optical properties of WSe2 monolayers (ML) by means of photoluminescence (PL), PL excitation (PLE) and Raman scattering spectroscopy at room temperature and as a function of hydrostatic pressure up to ca. 12 GPa
We have performed a comparative study of the pressure dependence of the optical transition energies corresponding to the radiative recombination of the A and B-excitons for a bare WSe2 ML and a hexagonal boron nitride (hBN)/WSe2-ML/hBN heterostructure, both transferred onto one of the diamonds of the diamond anvil cell (DAC)
Summary
Monolayers (MLs) of group-VI transition metal dichalcogenides (TMDs), MX2 with M=Mo, W and X= S, Se, Te, have emerged as fascinating twodimensional (2D) semiconductors due to remarkable properties that differ from the bulk. The strong spin-orbit coupling is responsible for large splittings in both valence and conduction band with several hundreds and tens of meV at the K points, respectively This gives rise to a large variety of optically bright and dark excitons, governing the optical properties from cryogenic to room temperature. Narrower PL linewidths have been reported for hBN encapsulated MLs. In this work, we show that different stress situations hold in the two samples, being fully hydrostatic for the Figure 1: (Left panel) Optical image of the DAC loaded with a single-layer of WSe2 (plus some bulk and few layers) at a pressure of ca. This has direct impact on sign and magnitude of the pressure coefficient of the A and B excitons, for example
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